Most people here (including me) aren't in the industry, so this may not be the place to ask for practical advice.  However you aren't alone in wanting to get into this from another field.

I would say think about what you want to do and think about what your differentiator is.  The principal areas where the industry (as it is perceived by a customer) is short of expertise is design/specification/configuration.  The actual plumbing is pretty standard plumbing which I would think any half decent plumber can do, and the electrics are simple.  I have an engineering/physics background and, if I were getting into this, Id try first to team up with a plumber and electrician, do the design, sales and commissioning, and bring them installation work.  I suspect you do need actually to do a bit of the plumbing so you understand the practical challenges that are faced onsite (unless you already do this as DiY).  Thisk about a heat geek qualification and MCS.  Your differentiator could be that you actually understand it, which is not true of all installers or people in the industry.

The other angle is streamlining, there is a need in my opinion to think about how we are going to get to 1.5M heat pump installs per year (currently we are at about 100,000).  It cant possibly be done the way we currently do it.  it needs some inovative products and some techniques I suspect.  However that needs development money.

Finally there is clearly a market for 'fixing' poor installs.  However I doubt there is any money in it.

Just some starting thoughts, Im sure others will have further ones.

Try to find a very good company that can teach you the basics in general heating and plumbing, try to then get on a couple of courses that specialise in design of low temperature heating design (but not the pretend 3 day course) something that you can apply, then try to get a little experience in buildings and building fabric, this will help with surveys, once you understand all that try to get as many supplier courses under your belt, these vary from 4hr very basic to 3 days intense applications.

In my experience 34 years in commercial and domestic Mechanical HR ventilation, AC, Gas, controls and Electical the very best way and the very best engineers often come from enhanced and well supported training apprenticeships or traineeships. Our Government has never really directly supported enhanced training in the HVAC world and after 19yrs old everything costs maybe £15K for training over a couple of years but sometimes there are companies willing to invest and take a chance on training some people from scratch.         

This is probably obvious (and applies to many spheres) but don't believe anything anybody tells you unless either they can explain it or you can understand why it might reasonably be true. 

You have the background to question both the 'received wisdom' and the pure chancers.  There are lots of knowledgeable people out there (and also here), but also some BS merchants.  Don't be fooled by the latter, you will quickly work out who they are.

@lakey, that’s a really solid foundation to build from, especially with your background in thermodynamics and fluid dynamics. A lot of people enter the heat pump space from the plumbing or electrical side and have to learn the theory as they go, but you’re starting with the technical understanding that many installers never fully get to grips with, even after years on the tools. That IMO gives you a serious edge when it comes to system design and diagnostics, which is where the real long-term value is in this industry.

If you’re serious about making the switch, you don’t necessarily need to go down the traditional plumber or electrician apprenticeship route unless you want to be a full-scope installer. What might suit you better is to aim for system design, commissioning or even becoming a heat loss specialist or technical consultant. These roles are becoming more and more essential as the push for quality installs ramps up, and they’re often lacking people with a proper engineering mindset who can look beyond “what we’ve always done.”

That said, getting some practical experience (either by shadowing an installer or doing short practical courses) would be really useful so you understand what it’s like on-site. Heat Geek’s design and advanced courses could be worth a look, and there are other providers doing solid training, though quality varies. Some manufacturers like Ideal, Vaillant and Mitsubishi run good technical courses too.

It’s worth remembering that most of the industry right now is crying out for people who can do more than just fit boxes to walls. If you can apply your existing knowledge to help improve design quality, performance and troubleshooting, there’s a lot of demand for that.

If you’re interested, I can put you in touch with installers in your area who might be a good fit for you to tag along with on a project or two to see what it’s like.

@editor Mars, thank you. That's along the routes I was thinking. I had considered reaching out to companies advertising system design engineers but they currently all state plumbing/electric background on the advert. 

If you can put me in touch with installers in the Manchester area willing to entertain the idea that would be fantastic. 

I've completed some of the free courses and content, I will have a look for the design courses.